Pet-1 is a new ETS domain transcription factor and is a rare example of a factor that appears to be expressed in a single brain neurotransmitter system. Pet-1 is expressed in the brain virtually exclusively in the 5-HT system. Preliminary analysis indicates that Pet-1 is expressed in most or all raphe nuclei, including the laterally extending B9 cluster. The sites of Pet-1 expression in the reticular formation map very well to the 5-HT neuron clusters classified as the B3 group. Thus the pattern of Pet-1 expression in the brain is limited to most and perhaps all 5-HT nuclei in the brainstem. Based on the likely role of Pet-1 as a sequence-specific transcriptional activator and its provocative expression pattern, it is hypothesized that Pet-1 functions to control either the specification, proliferation, or differentiation of serotonergic neurons in the vertebrate CNS. The detection of Pet-1 RNA in E14 rat head is consistent with a function early in development. Pet-1 may be a precise marker for all or part of the 5-HT neuron lineage. It may mark the earliest serotonergic neuron progenitors or cells at a later stage in the differentiation of the lineage. The possible developmental role of Pet-1 will be investigated by determining when and where it is expressed in the fetal brain. In situ hybridization and immunohistochemistry will be used to precisely determine the expression of Pet-1 in the 5-HT system. The onset of Pet-1 expression will be determined and compared to the appearance of 5-HT in the brainstem. Pet-1-specific polyclonal antisera will be generated to provide an immunological reagent for the detection of Pet-1 in the brainstem and for biochemical analysis of its function. The proposed experiments will suggest likely functional roles of Pet-1 in the developing brainstem and will facilitate interpretation of mutant phenotype in future gene targeting of the murine Pet-1 locus. The identification and characterization of a transcription factor that is likely to be a key regulator of the vertebrate CNS 5-HT system has clear significance for elucidating the molecular mechanisms governing the development of this vital neurotransmitter system. This system is involved in the control of numerous neural functions such as cognition, affect, pain, appetite, sex, sleep, aggression, and perception. Abnormal function of the 5-HT system has been implicated in numerous psychiatric disorders. Future loss or gain of function approaches to the study of Pet-1 in the mouse may create novel models for clinical disorders involving the 5-HT system.